In the final chapter of The Adventures of Huckleberry Finn, Huck announces he's going to continue his travels to escape Aunt Sally's attempt to civilize him. To some readers, this might seem to signify a lack of fulfillment or atonement. After all, some people might think that, if Huck were fulfilled by his journey, he would logically stop traveling. While you certainly could argue that point, it's not necessarily the proper perspective to take with this book. Indeed, it's possible to see Huck's determination to continue wandering and avoid becoming "civilized" as its own kind of fulfillment.
Consider, for instance, that Huck spends the bulk of the novel witnessing the corruption of society. From the racist institution of slavery to the heartless, swindling schemes of con men, Huck experiences the full spectrum of humanity's evil and vice. By the end of the novel, one could justifiably question whether the depicted society is "civilized" at all. As such, Huck's decision to continue wandering signifies a sort of fulfillment, as it is a decision that logically follows from and responds to the earlier experiences of the novel. Indeed, Huck's realization that maintaining one's freedom and personal dignity often requires separation from society and civilization is perhaps the clearest sign that he has been fulfilled by his journey down the mighty Mississippi.